Add the sausage meat and onions to a skillet. Sauté for about 7-8 minutes, breaking the sausage meat up with your spoon, until browned. If needed, spoon out some of the excess fat.
Stir in the garlic and cook for about 30 seconds. Add the sausage mixture to your Crockpot.
Add the carrots, celery, canned tomatoes, and chicken broth to the Crockpot. Keep in mind that if you want the soup to cook in 3-4 hours, the carrots will have to be chopped pretty finely. If you're cooking it for 7-8 on low or 4-5 hours on high, you can chop them larger.
About 30 minutes prior to serving, add in your cheese tortellini, cream, and spinach (I set the heat to "high"). Give it a stir occasionally. Season with salt & pepper as needed prior to serving.